The regulation of steroidogenesis by the adrenal cortex is a complex process involving the transduction of signals from the plasma membrane to specific intracellular sites, protein phosphorylation, protein activators and/or inhibitors, cellular uptake and disposal of cholesterol, cholesterol synthesis and metabolism, intracellular steroid transport mechanisms, and steroid secretory processes. The rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone, a process whereby cholesterol must be made available and delivered to and pregnenolone removed from the active site located in the inner mitochondrial membrane. To study this complex process in detail, the guinea pig, a cortisol producer like the human being, has been employed as an animal model. A. Plasma membrane adenylate cyclase. Adenylate cyclase activity and cyclic AMP production has been determine using isolated cells and membrane particles. B. Cytoplasmic cAMP-dependent protein kinase. Cyclic AMP-dependent protein kinase activity was found mostly in the soluble fraction (80-90%), composed of the type II isozyme, and 40% more active in the outer cortex. A study of soluble and particulate phosphoproteins is in progress. C. Binding of lipoproteins to membrane receptors. Low density lipoprotein receptor activity and response to hormonal and drug manipulations was measured. D. Cellular uptake of cholesterol and cholesteryl esters. Labeled compounds injected intravenously incorporate into circulating lipoproteins and are taken up by various tissues of which the most avid is the adrenal cortex. E. De novo synthesis and metabolism of cholesterol. The activities of enzymes HMG-CoA reductase (rate-limiting in cholesterol synthesis), acetyl CoA: cholesteryl acyl transferase, and cholesteryl esterase are being investigated. F. Specific steroid-binding proteins. A specific pregnenolone-binding protein (PBP) is being further purified using high performance liquid chromatography. Preliminary data suggest that the PBP is phosphorylated by a cAMP-dependent kinase.